1. Field of the Invention
The invention relates to semiconductive solar cells and somewhat more particularly to solar cells composed of semiconductive materials having an active zone provided with an electric insulating coating with metal contacts therein so that charge carriers are produced in the active zone via energy striking and penetrating into such cell.
2. Prior Art
As an alternative to conventional solar cells having a flat pn-junction parallel to a semiconductor surface, the so-called MIS solar cell (metal-insulator-semiconductor) is known. In such devices, pairs of electron holes produced by the light energy are separated in an electric field of a Schottky contact, where metal and semiconductor are separated by a thin insulating layer (having a thickness of less than 5 nm). In instances where the semiconductive material is silicon, the thin insulating layer is a SiO.sub.2 layer. In comparison to normal Schottky contact solar cells, this oxide layer, through which electrons can tunnel, is characterized by a higher open-circuit under illumination without substantial increase in the series resistance of the cell. A MIS solar cell of this kind is disclosed for example, in an article by R. J. Stirn and Y. C. M. Yeh in "Applied Physics Letters," Vol. 27, No. 2, July 7, 1975, pages 95-98.
Advantages of such MIS solar cells, in comparison with conventional pn-cells include:
(a) no high-temperature manufacturing steps and thus lower production costs, in comparison with pn-cells;
(b) no crystal structure defects resulting from diffusion, which structures can act as recombination centers and can thus substantially reduce the efficiency of a device;
(c) higher efficiencies can be achieved for short wavelengths (UV) since the electric field extends to the semiconductor surface and consequently the absorption of the UV-light occurs in a zone of high field strength;
(d) a high radiation resistance;
(e) more adaptable for use of polycrystalline and amorphous semiconductive materials so that cheaper devices are attained.
However, difficulties are achieved in producing a uniform large-area MIS contact, including the required extremely thin light-permeable metal layer.
Another alternative in attaining relatively cheap solar cells which at least partially avoids the above mentioned difficulties, consists in using a so-called inversion layer solar cell. In such devices, the charge carriers are accumulated by using a conductor path grid composed of a relatively thick metal layer with an underlying pn-junction, such as disclosed by G. C. Salter and R. E. Thomas in "Solid State Electronics" Vol. 20, 1977, pages 95-104, or a MIS Schottky contact such as disclosed by P. van Halen, R. Mertens, R. Van Overstraeten, R. E. Thomas, J. Van Meerbergen in "Proceedings of the European Photovoltaic Conf." 1977, pages 280-288, D. Reidel Publishing Co., Dordrecht, Holland/Boston, U.S.A. The area between the metal paths is coated with a transparent dielectric layer which can simultaneously function as an anti-reflection layer. Fixed insulator charges on the interface between the insulator and the semiconductor produce an inversion layer (composed of minority charge carriers) followed by a space charge zone in the semiconductor, directly beneath the dielectric layer. The inversion layer forms one side of an induced pn-junction in which a high electric field exists and which promotes the collection of the photo-charge carriers. The resistance of such inversion layer is comparatively low; in such thin surface inversion layers, the minority charge carriers produced by the light diffuse, as in a metal film, toward the contacts where they are discharged.
Similar to the above-discussed MIS cells, inversion layer solar cells must also exhibit a higher UV sensitivity than pn-cells because of the lower surface recombination velocity. In order for the conductivity of the inversion layer and thus for the photo-current to be as high as possible, the interface charge density (Q.sub.SS) must be as high as possible and at the same time the density of the fast surface states (N.sub.SS) must be low. The fast surface states reduce the conductivity due to the capture of minority charge carriers (recombination).
Accordingly, the main requirements for inversion layer solar cells are:
(1) a high density of fixed interface charges in order to achieve a highly conductive inversion layer;
(2) a low density of fast surface states in order to keep the loss (recombination) of charge carriers low.
When a thermally produced oxide is used as a dielectric on silicon, apart from the disadvantageous use of high temperatures of above 1000.degree. C. (power consumption, crystal defects, etc.) which are required to form such a layer, an increase in interface charge density, Q.sub.SS and thus a reduction in the layer resistance of the inversion boundary layers is always coupled with an increase in the density of the fast surface states, N.sub.SS. The increase in Q.sub.SS can be achieved by annealing in oxygen at relatively low temperatures and by selecting (111)-silicon in place of (100)-silicon as a substrate material. In extreme instances, Q.sub.SS /q values of approximately 1.times.10.sup.12 cm.sup.-2 are achieved together with very high N.sub.SS values of approximately 10.sup.12 cm.sup.-2 eV.sup.-1.